Expendable Pattern casting, also known as lost foam casting, is a known casting technique in which a pattern formed of an polymeric foam material, such as polystyrene or polymethylmethacrylate, is supported in a flask and surrounded by an unbonded particulate material, such as silica sand. When the molten metal contacts the pattern, the foam material decomposes with the products of decomposition passing into the interstices of the sand while the molten metal replaces the void formed by the expended foam material to produce a cast part which is identical in configuration to the pattern.
In the conventional expendable pattern casting process, the sand which surrounds the pattern and fills the cavities in the pattern is unbonded and free flowing and this differs from traditional sand casting processes, wherein the sand is utilized with various types of binders. However, after compaction, the unbonded sand density is generally higher than the density of molds made with bonded sand, and therefore the rigidity or stiffness of compacted unbonded sand is not deficient relative to bonded sand molds.
Traditionally, silica sand has been used exclusively as the molding material in expendable pattern casting because it is readily available and inexpensive.
Aluminum silicon alloys have been cast utilizing expendable pattern casting techniques as disclosed in U.S. Pat. No. 4,966,220. Aluminum silicon alloys containing less than about 11.6% by weight of silicon are referred to as hypoeutectic alloys and the unmodified alloys have a microstructure consisting of primary aluminum dendrites, with a eutectic composed of acicular silicon in an aluminum matrix. Hypoeutectic aluminum silicon alloys have seen extensive use in the past but lack wear resistance.
Hypereutectic aluminum silicon alloy, those containing more than about 11.6% silicon, contain primary silicon crystals which are precipitated as the alloy is cooled between the liquidus temperature and the eutectic temperature. Due to the high hardness and higher modulus of the precipitated primary silicon crystals, these alloys have good wear resistance but are difficult to machine, if the primary silicon particle size is large, a condition which limits their use as casting alloys. These alloys also have a high-cycle fatigue strength, 50% higher than typical hypoeutectic aluminum-silicon alloys, because hypereutectic aluminum-silicon alloys do not contain the primary aluminum phase associated with hypoeutectic aluminum silicon alloys. These higher fatigue strengths, however, have not been utilized in practice because hypereutectic aluminum-silicon alloys are not used commercially in sand casting processes, such as expendable pattern casting.
Normally, a solid phase in a "liquid plus solid" field has either a lower or higher density than the liquid phase, but almost never the same density. If the solid phase is less dense than the liquid phase, floatation of the solid phase will result. On the other hand, if the solid phase is more dense, a settling of the solid phase will occur. In either case, an increased or widened solidification range, which is a temperature range over which an alloy will solidify, will increase the time period for solidification and accentuate the phase separation. With a hypereutectic aluminum-silicon alloy, the silicon particles have a lesser density than the liquid phase so that the floatation condition prevails. Thus, as the solidification range is widened, the tendency for floatation of large primary silicon particles increases, thus resulting in a less uniform distribution of silicon particles in the cast alloy. Conversely, if the rate of cooling through the solidification range is increased, the tendency for floatation of the primary silicon particles is decreased resulting in a more uniform distribution of smaller silicon particles in the alloy. However, at sand casting cooling rates, improvements in wear resistance or machinability by using different sand types, have not been recognized.
It is recognized in the casting art that using a molding material that extracts heat more rapidly from the molten metal and allows it to solidify at a faster rate, yields a casting with superior mechanical properties. A cooling rate increase of three orders of magnitude (i.e. a 1000 times increase) decreases the dendritic arm spacing of the primary aluminum phase of hypoeutectic aluminum-silicon alloys by one order of magnitude (i.e. a factor of 10). This microstructure change results in an increase in mechanical properties. Thus, castings produced using metal molds, which extract heat rapidly, generally exhibit superior mechanical properties as opposed to castings produced by sand casting or expendable pattern casting processes that utilize sand as a molding material. However, when using sand as a molding material, as in sand casting or expendable pattern casting, doubling the cooling rate (which is theoretically the most that can be expected from the higher heat diffusivity obtainable with any sand media), decreases the dendritic arm spacing of hypoeutectic aluminum-silicon alloys by approximately 10% and this reduction results in only a 5% increase in the ultimate tensile strength. Thus sand casting properties of hypoeutectic aluminum-silicon alloys are never listed in the reference books by sand type.